Controlled enhancement of electronic conductivity in highly isolating thin molecular membranes by new types of intrinsically conductive organic or organometallic compounds is highly in demand, because this gives possibilities for improving the sensitivity and selectivity of electrochemical processes occurring at the solid/liquid interface [Atta N. F. et al. (1991) Biosensors and Bioelectronics 6, 333-341; Hable C. T. & Wrighton M. S. (1993) Langmuir 9, 3284-3290].
Generally, for application in chemical sensing devices, conductivity is desired through thin molecular membranes of different types, including biological membranes, biomimetic membranes and thin polymer films, with a thickness varying between 50 and 100 .ANG., [Merz A. (1990) Top. Curr. Chem. 152, 51-90; Ottova-Leitmannova A. & Tien H. T. (1992) Prog. Surf. Sci. 41/4, 337-445]. Additionally, the introduction of electronic conductivity in highly isolating bulk polymers is actively studied, particularly for electronic shielding purposes [Cao Y. et al. (1993) Synth. Met. 55-57, 3514-3519].
Transport of electrons may be induced by, in principle, three types of mechanisms, as illustrated in FIGS. 1a-c, where an electrolyte solution is denoted by 1, an insulating membrane by 2, a metallic electrode by 3, electron transfer by arrow e, and an electron donor (reductor) by "Red".
FIG. 1a shows schematically the transport by electron-mediation, in which a hydrophobic electroactive species 4, the "mediator", receives one or more electrons at one side of the isolating membrane 2, subsequently diffuses (arrow d) to the opposite side of the membrane and donates the electron(s) to the electrode. The oxidised mediator is denoted by a black dot and the reduced mediator by a minus sign. Examples, known in the art, are the incorporation of certain small dye molecules or highly conjugated organic molecules into bilayer lipid membranes [Janas T. et al. (1988) Bioelectrochem. & Bioenerg. 19, 405-412; Kutnik Y. et al. (1986) Bioelectrochem. & Bioenerg. 16, 435-447].
FIG. 1b shows schematically the transport by electron exchange (`hopping`), using a molecule that contains multiple electroactive species (a chain 5 of redox centres), fixed at predetermined distances from each other by a chemical tether. In this system electrons are received at one side of the membrane and jump between the electroactive centres across the membrane.
FIG. 1c shows schematically the transport by electron-delocalisation, using a molecule 6 containing an extended .pi.-electron system, capable of receiving electrons at one side of the membrane 2 and delocalising the electron nearly instantaneously to the other side of the membrane by a quantum-chemical mechanism. Such molecules are acting as a direct electronic wire between an electroactive species and the electrode surface.
In the first mechanism, of FIG. 1a, electron transfer rates are largely controlled by the diffusion of the electroactive species through the membrane, which is a slow process. With multiple redox centres the process of conduction, as depicted in FIG. 1b, is controlled by the self-exchange rate of the redox species, the mean distance between the redox species and the dielectric constant of the membrane phase. This is generally a much faster conduction mechanism than that of mediated electron transfer.
In the last case, with an extended .pi.-electron system, as illustrated in FIG. 1c, electrons are delocalised over the whole length of the molecule, a conduction process that is about 5 orders of magnitude faster than that of conduction through redox chains. Electron transfer with these types of molecules is in principle the most effective. With membrane thicknesses below 40 .ANG. an additional conduction mechanism may occur, namely that of electron tunnelling [Thompson D. H. P. & Hurst J. K. (1988) in: Carter F. L. et al. (Ed.) `Molecular Electronic devices` Elsevier, Amsterdam, 1988. pp. 413-425].
In earlier studies by other workers it has been shown that carotenes, modified with terminal pyridinium groups, "caroviologens" (structures according to formula II), may be incorporated in liposome bilayer lipid membranes in a characteristic through-membrane orientation, which is a main prerequisite for effective electron transfer through the membrane [Arrhenius T. S. et al. (1986) Proc. Natl. Acad. Sci. 83, 5355-5359; Johansson L. B.-.ANG. et al. (1989) J. Phys. Chem. 93, 6751-6754.]. ##STR2##
Although the caroviologens are functionally active as molecular conductors, the conductivity change, observed in liposomes, was not very large: only a 4 times increase of electronic conductivity was observed for an alkyl sulfonated caroviologen derivative [Lehn J.-M. (1991) in: Schneider H.-H. & Durr H. (Ed.), `Frontiers in Supramolecular Organic Chemistry and Photochemistry` VCH Publ, Weinheim, 1991. pp. 1-28]. Further research on more optimal conductors has mainly been concerned with further modification of the terminal groups of the polyene chain [Blanchard-Desce M. et al. (1988) J. Chem. Soc., Chem. Commun., 737-739; Thomas J. A. et al. (1992) J. Chem. Soc., Chem. Commun., 1796-1798; Bubeck C. et al. (1992) Adv. Mater. 4, 413-416].
It is known, however, that the chemical stability of the polyenes, and particularly the carotenes, is not very high. Also polyene structures exhibit photochemical cis-trans-isomerization, which affects the planar conformation of the molecule and the incorporation in thin organic films [Carter F. L. et al. in: Carter F. L. et al. (Ed.) `Molecular Electronic devices` Elsevier, Amsterdam, 1988. pp. 465-481]. Particularly the coplanarity of the whole .pi.-electron system, is an important condition for efficient conduction.